A search engine is a program that helps users find relevant information on a connected set of information sources. While the information sources can be connected by any mechanism, we will assume in this document—only for the sake of providing a concrete example—that they are on the World Wide Web. Numerous web search engines exist today. In this section, we first briefly survey the state-of-art of search engine technologies. We then discuss our methods for extending existing search engine technologies for searching for fine-grained objects and related entities, as well as some example applications of such a search engine.